Process for the preparation of cellulose nitrate



meme Jan. 16, 1945 UNITED s'rA'rEs PATENT OFFICE- PROCESS FOR THE PREPARATION OF CELLULOSE NITRATE williamjE. Sillick, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application January 23, 1941, Serial No. 375,650

7 Claims.

This invention relates to process and apparatus for the recovery and concentration of inorganic acids and moreipartlcularly to the separation and concentration of nitric and sulfuric acid mixtures I obtained from cellulose nitration.

As is well known in the industry, large amounts of nitric acid, usually in conjunction with sulfuric acid are employed. These materials are thereafter obtained in the form of waste solutions and in order to reutilize the acids they must be recovered and concentrated.

In more detail, for example, in the nitration of cellulose at the end oi the reaction time it is [necessary to separate the mixture of nitric acid and sulfuric acid from nitrated-cellulose. At the.

present time the method commonly practiced is centrifuging in a machine oi a perforated basket type. This type of process will reduce the amount of acid remaining on the cellulose nitrate to, for example, the same weight as that or the cellulose nitrate. At that stage it has theretofore been considered necessary to quickly submerge the cellu- 1 lose nitrate in water or some other liquid to prevent firing of the unstable product and to so dilute the acids remaining on the product that either nitration or denitration reactions stop.

separation and recovery of waste nitration acids which is more eflicient and satisfactory than prior art methods. Still another object is to provide a method whereby the recovered acid may, be further concentrated. Another object is to provide novel apparatus for carrying out the aforedescribed processes. Still another object is to provide a'novel apparatus wherein the steps may be conducted in a substantially continuous and an automatic manner. Other objects will appear hereinafter. a

I have found that contrary to the prior art procedure, where extremely large amounts of which are already in a substantially stronger con-- The general practice is to submerge the solu-- I tion in sumcient water that the suspension of cellulose nitrate in water which will flow by gravity through conduits, is formed. Bysuch submerging in large quantities of water it is readily .apparent that the acid value is substantially diluted to, for example, 3 percent acid. It is readily apparent that the removal of water from such low concentration acid by evaporation would require the expenditure of large quantities of heat and otherwise present substantial problems.

I have found an improved process and appare tus for carrying out the procedure whereby the acid not only may beseparated and recovered as relatively concentrated solution, for example,

five to ten timesas concentrated as heretofore om tainable in the prior art, but I have further .found that the partially concentrated acid may be still further concentrated to substantially anhydrous old as, for example, 95 percent nitric acid by my new method described herein.

'liiis invention has for one object to provide a method for the separation and recovery in at least apartially concentrated condition of inorganic pacids such as nitric and sulfuric acid. Another object is tcprovide a novel method of recovering inorganic acids from reactlonmaterials whereby any injury to the reaction product is prevented.

' Still another object is to provide a method iorthe c'entration than has hitherto been obtained, may be still further concentrated by a distillation procedure which includes reflux, as will be described in detail hereinafter.

For a more complete understanding of my in I vention reference ismade'to the attached drawing forming apart of the present application.

Figure 1 is a semi-diagrammatic side elevation view oi one embodiment of a novel apparatus wherein my recovery and separation steps may be carried, out. Certain 'parts have been shown broken away or on an exaggerated scale for clarity. Figure 2 is likewise a semi-diagrammatic side elevation view of suitable apparatus which may be employed for carrying out certain of the acid concentration steps ofmy invention. a

Referring now to Figure 1, my equipment may be considered as comprising four units in series, indicated as Nos. 1, 2, 3, and 4. These units would preferably be providedwith one or more agitating means, designated as Nos. 6, 7, and 8, for accomplishing a thorough mixing of the solids involved in the process with the liquids in each unit. Connected with the first unit of the series is the feed basin 9 which discharges through the feed conduit H into unit l. Within-unit l (as well as the other units) is provided a screen conveyor I 2. This conveyor can be considered as v and otherwise a I accuses" squeeoe rolls it, ",and II, provided at the other end of the screen conveyor.

Unit I is provided with a sump ll which serves to collect the liquid irom unit I. At one point adjacent this sump may be provided a drawotf eenduit is which" discharges into the container 28. At a lower point. on thissump is ,provided another conduit N which, for example, may feed through pump it io'the recirculation system which may be considered'as starting at cone duit 21. Y

Conduit 21 may include in its circuit means for and the temperature controlling device, which may may comprise an ordinary tube bundle in the na of a condenser or heat exchanger, is

not alimitatiori upon this invention. Hence a detailed description is unnecessary. It is of course understood, for example, that the unit at, I should have sufilcient capacity to control the temperature or the liquids passing therethrouirh within the deaired'temperatureranges discussed hereinafter under an explanationof the operatiou or my novel process. The cooling medium may well be the water which will later be used to stabilire the nitrocellulose; and could be subsequently fed-inst points II and II.

The conveyor means it or unit I may be considered as dischargingthrough rollers II and" Hernia ems water at Ii. Provision is made at! for sbvapor takeoi! conduit to'whichacid fumes and may be evacuated. At this point itmaybe men-.

I tioned tha'tif desired similar exhaustconduits.

could be'provided. for each separate" unit. As

hown in Figure}, the various units may 'be'interoonnected with conduits such as design ted at 84,-", and 5.. mother words. any, suitable l0. provision may be-made for the exhaustion of fumels'which may be present in the various units. Unit ldischarges through rollers 51 and 88 into the closed chamber l0. Provision is made at' II for the introduction of water and at II for is the withdrawal or the materials; I i

, It will be observed that in rigure 1 the various units have been placed, not on the same level but, each succeeding unit or the series at a slightly raised-level; As is, apparent rroml'igure 1, b "so ch arrangement and, also, inclination of the reen conveyors, the liquid flow through the apparatus'can be caused to'takexplace by gravity.

It is, of ooursc,-t be understood that if gravity I flow were notdesired or could not bearranged, as the units could be-placed on the same level and pumpsbe employed tocause the'transfer of the liquid from onechamber to anothen However;

. because 0! simplicity, gravity flow-as in fthedrawinmisnreferred.

' It is understood'that. the 'apparatua since 1; a 7 in contactv with acid and acid fumes at various points, would be'c'onstructedi of materials which would withstand corrosion. For example, some of the materials, which may be employed in the into unit which is separated therefrom by, for construction or my apparatus are austenitic example, partition means ll. Unit! is or subhas just been described in that it includes a con veyor means ll,- a sumpv 31 (preferably having 4 ceramics may be employed, The foregoing matepositioned therein agitator'nieans which has al-.

ready been referred to) squeeze rollers 38 and I9,

similar to unit I already described in detail. However, inthe case-o! unit 2, as well as the other units in series therewith,

the discharge orifice ll, rather than discharging into an external container, would discharge into unit I. Another point of diflerence is that the withdrawal from sump 31 through conduit ligand pump 48 is carried through conduit 44 back into unit 2 so as to spray or otherwise discharge onto convey r It,

Referring now to unit a this unit is likewise imilar to the units described in that it includes a screen conveyor designatedin its entirety as 48, a sump 41, overflow 48 and withdrawal means comprising conduit I! through pump as. and conduit It for discharging back to chamber 3 over conveyor ll. In add tion'to unit I there could be placed still other units in series which would be of construction identical with unit 3. However, in the particular apparatus under consideration. wherein only four units in series have-been described, the

final unit, unit 4 in this instance, will now be considered. It is to be understood, however, that the exact number of units in series is not a limi-. tation upon my invention but that, for example.

the number may vary over w de limits as three rials are obtainable commercially and their am y position is described :in publications as for example Koroseal is discussed in Ellis publication on resins.

I t islikewise to be understood that valves may be inserted at various points in the apparatus or other controlling device for assisting in the carryiing out of various processes in the apparatus. For example, it will be observed that in connection with filters is and", valves may be inserted a so that one filter may be cut out or operation,

cleaned or repaired while the other. filter is in operation, It is to beunderstood thatsuchfeatures may be employed with respect to control Si by, for example; placing a plurality oi temperature controllers in parallel-with other changes'incorporated. Figure 1 has been described primarily for generically illustrating the.

preferred embodiment of my apparatus and is not to'be considered as an undue limitation upon my invention. i

Referring to Figure 2, there is here disclosed a distillation column II which may be of conventional design. That is, the columnmay be either a packed or plate column. The lower portion of the column is connected with, for ex: ample, the jacketed still pot II. If desired, other methods of indirect heating as, for examples .and seven units have been found to be satisfacmany parts: hence. for conciseness only the parts units to ten or more. lln general. between four still not may be drawn on.

- The upper portion of the column is connected to a' vapor oirtahe conduit 14 which leads throulh 'a condenser or other cooling means I! to a weir:

The last unit in the series, unit I; i'slikewise substant ally similar to the preceding units in iacketed coil could be employed A suitable draw- I oi! 13 is provided so that the contents or the m this unit m m g nt p s in box llv or otherdevice for collecting the com The optimum value is 30%.

densate. A reflux conduit is provided at n an a drawoii conduit at II. Feed to the column is provided by means of conduit 1.9.

accuse describing my invention'in connection with con;-

- ditions actually encountered commercially. This The operation of my apparatus will be understoodfrom the description of the processes set forth hereinafter. o Referring now to the process of separating and recovering acid as, for example, the recovery oi" waste nitration acids in the manufacture of cellulose nitrate, as already indicated, in 'prior .art

practice such large amounts ofwater have been employed that these iwaste nitration acids have only been obtained as, for example, very dilute so-,

lutions of only 3 percent strength.

In my process I have developeda much more economical method whichwould, for example,

permit obtaining these waste nitration acid; in

approximately ercent strength by weight instead of the aforementioned 3 percent. In general my method consists of employing a countercursqueezed from the rent washing in order to obtain the acids in the state ofhigher concentration. "The cycle in each stage of my process may be considered as comprising" 1) introducing andmixing the cellulose nitrate and water in the stage; (2) allowing these materials to approch equilibrium conditions; (3)

separating the nitrocellulose from the water by gravity or applied forces; and (4) conveying to the next stage. In performingthis process I have found the -folowing factors to be of definite importance:

mixture of nitrocellulose and wast acid is disj' charged into the feed-basin 9 where it is so'used with the liquid, conducted through the. recircue The resultant through conduit 1| onto the screen 'c'onveyor l'h The material discharged onto the'eonveyor'isi' carried from the point of discharge upward on; v the conveyor toward the rollers l9 and 2||. Dur-. ing thistrip the liquid will drain through the screen conveyor away from the solid. The :ma-

terial is. discharged from the conveyor. through the rollers i9 and 20 where-frirtheniiquid. is A solid. rill of thisliquid collects in sump 2|.

The solid, which in the example eration is cellulose nitrate, falls into unit 2 where it is dispersed into th liquid therein, by meansof a stirrer 6. A portion of this liquid and nitrocellulose from unit 2 is drawn'from conduit l2- and pumped by means ofpump 4; to conduit 44 where it is discharged onto screen conveyor-.36.

(1) The combined mso. and'l-INOa must not be allowed to exceed by weight in the water.

(2) The temperature of the water-acid mixture of highest concentration must not exceed 130 F. The optimum figure is 100 F. or less.

(3) The cellulose nitrate must not remain in contact with the water-acid mixture of highest concentration for more than one hour, preterably not more than fifteen minutes.

.These three factors determine the amount of denitration which occurs during thecountercurrent process.

(4) The ratio of liquid on the cellulose nitrate after step No. 3 in the above cycle to the cellulose nitrate influences the emciency of the process and should not exceed 7:1 by weight, preferably notexeeed 3:1 by weight.

(5) The number of stages employed influences As in unit I the solid on conveyor 38 becomes separated from theliquid to'some extent, and'isdischarged through rollers 38 'and 39 whereiurther liquid is squeezed out. The liquid squeezed out remains in unit 2 and the-cellulose nitrate. is diso aked thro unit indicated as t.

In this unit the cellulose nitrate is againmixe with the liquid in the "unit, pumped to a pointabove the conveyor, carried on the conveyor, the

the emciency of the process. With the proper control of the process a recovery of 90 percent of the waste acid can be obtained in. 30 percent concentration by means of only three stages of coun-.

'tercurrent washing. However, it may be more economical in some cases to employ as many as 7 stages. I

Thus it is made possible, with little expenditure of energy, to recover the waste acids in sufllcient concentration that they can be'economically concentrated to usual strengths. r A still further detailed description of the application oi my process, employing the apparatus of Figure 1, will "be had from'the following description which will likewise concern the separation of waste nitration acids from nitrocellulose. Referring now to Figure l, the nitrocellulose from nitrating centrifuges in accordance with existing practice, probably would contain an amount of acid of approximately the same weight as the cellulose nitrate. It is to be unliquid separated and cellulose nitrate passed through the rollers into the next succeeding unit. By this manner the cellulose nitrateproceeds irom st seto stage until it is finally discharged However, in the l'astunit of the series, which in the example under consideration "is-unit 4,

water is supplied through conduit SI. 'lnsuch quantity that it overflows through orifice 59 from stage to stage until it finally leaves unit! through conduit 22 at the desired strength in acid, By

,siipplying sufiicient. water through conduit 5| the content of acid in' unit l'will not build up to exceed 40 percent as referred tounder (1) above.

'Likewls'e by pumping the liquid from unit I through fi ters and any cellulose nitrate ,iibersare removed and hence, the timeoi con tact with anycellulose nitrate fibers of [the strong-acid water inunit I is limited. By means of temperature controller ii the temperature or the strong-acid and water mixture introduced into the vessel 9 maybe held below the maximumspecifled by (2) above. By controlling'the speed of the. various pumps 28, 43, etc., as wellasthe quantity of cellulose nitrate fed, and the speed of the various conveyors i2, 35, etc.,'the period derstood that the exact amount of acid mixed with solid from which the acid is to be recovered and separated. is not a limitation upon my in vention but been referred to for more clearly I! lation line comprisedor parts 21, 2:8; 29; ii, and

mixture in flfcoiinprisingin the instance of the example under, consideration, a cellulose nitrate, acid, and water is discharged under consid ugh rollers 38 andlfliinto the 41 y Y aqueous acid liquid may be separated and recovered which in many instances is approximately ten times stronger than the aqueous acid which has heretofore been in comparable processes for the preparation of cellulose nitrate.

- .-While such 30 percent acid may be usedfor some purposes as obtained, in many instances it may be desired to concentrate the acid further as, for example. to a 95 percent nitric acid. As-

- suming that thenltric acid is mixed with sul- "iuric acid as in many instancesencountered in industrial operations, it may also be desired to efiect a separation .of the acids.

Prior'to my invention in concentrating nitric acid solutions by distillation, ithas been suggested to eliminate the water presentwith nitric acid byemploying a dehy rating ent. Such procedure has been considered'required because of the fact, that nitric acid forms a maximum' I boiling-mixture with water of the composition 68 percent nitric acid-82 percent water. Hence, itv would appe'arthat the dehydration of nitric acid by simple distillation would be impossihlebeyond a 88 percent concentration; I

In prior methods which have been suggested for concentrating nitric acid. to-my knowledge no one has appreciated that not only should the admixed dehydrating agent. H2804 be introduced at the top of the-column but that in some cases it is alsobeneilcial to return a portion of the distillate as a reflux to the top of the column.

' I have found it possible by this method to obtain continuously a product in nitric acid as rich as, or richer than, the vapor in equilibrium with the i'ee'd mixture .atits boiling-point. The advantages of this operation will be revealed later..

In my process, as I have already indicated, it is immaterial that the nitric acid from the'procass of Figure 1 contains sulfuric acid.

Reierringn'ow to Figure 2 which has already.

usual strengthsof weak 8180s. This permits operatingthe ENC: concentrator at a lower tem-' perature and also permits reconcentrating the H2504 at a lower It can be seen from the foregoing, therefore, that I have provided novel procedure and apparatusfor permittingthe irecovery of inorganic acids and in particular nitric acid in either partially concentrated condition such as a percent acid or an concentrated acidsuch as a 05 percent acid from solutions which heretofore have either been lost as waste or presented extreme diflicult'ies of concentration by attempting to evaporate, for example, substantially all of the waterfrom a 3 percent acid solution. My process may be operated more or less automatically and substantially continuously. whlle l have described my invention as applied to acid mixtures of particular concentrat'ons which correspond to concentrations obtained industrially. it has been primarily for the purpose of illustration and it is to be understood that my invention is not to be restricted in this respect. My invention been described and may comprise in its con;

'struction a column of conventional design and other conventional parts, the dilute acid, as for example a 30 percent acidof the nature referred to, after a simple rectification process to remove some of the water,'-would be fed into the distillation column through conduit 1s along with suitable amounts or 11280 at least 80% strength.

Heat is applied to the base heater I2 by means of the steam Jacket and distillationconducted .in standardmanner; The vapors which pass through'vapor off-take conduit 14 are led through condenser- II and a, nitric acid .condensate is caused to collect in the weirbox It. A portion of this nitric acid condensate is returned-to the column through conduit IT as reflux. The other portion may be drawn off through conduit II as a concentrated nitric acid for whatever-use is desired. In operation of my process, for-example, a column oi between 4' and 15 plates could be employed. The proportion of the condensate refluxed wouldvary between about .20 percent and 80 percent of the'condensate at ll.

may be applied to various mixtures of inorganic acids containing various amounts of nitric acid and sulfuric acid, either from waste solutions or otherwise,it being kept in as pointed out above, that in the distillation step of Figure 2, the mixture would be brought into certain limits by the addition of percent or higher sulfuric acid to make the feed supplied through conduit It. It is also apparent .fromthe foregoing that certain changes may be made in my invention as, for example, modifications of the treatment of the 30 percent acid after. recovery by the countercurrent process and before flnal distillation, or use In the prior art methods for distilling nitric acid, as I have indicated, it has been customary to distill the nitric acid in the presence of concentratedsulfuric acid a which. retains the water, permitting the nitric acid to .be distilled off and condensed-directly; no reflux-being used.

However, the advmtages of my novel method of operation are several. It can be applied to aqueous (diluteor partially concentrated) feed.

mixtures which are impractical for-use in pros,- eases commonly used-today. For example, it is possible to obtain continuously with good yield of conventional methods of BNO: concentration in conjunction with the countercurrent process. or lay-reusing the 30 percent acid in conjunction with an ammonia oxidation plant as a means of supplying water to the scrubbing towers. Also while I have described the use of my countercurrent treatment and apparatus on "nitrates which present special problems of decomposition,

my apparatus may be used on other materials. That is cellulose esters or mixed esters may be washed therein. Hence, I do not wish to be restricted excepting insofar as is necessitated by the prior art and the spirit of the appended claims.

I claim:

1; In the mamiiectureoi cellulose nitrate-in which celluloseis nitrated and then centrifuged.

,the steps which comprise immediately sousing the cellulose nitrate. after centrifuging. in a liquid essentially consisting oi an aqueous mixture of nitric and sulfuric acids having an acid '7 concentration of 25-40%, and a temperature of below separating the nitrate therefrom and least approached in each wash liquid which is employed.

In the manufacture of cellulose nitrate in which cellulose is nitrated and then centrifuged,

the steps which comprise immediately sousing the cellulose nitrate, after centrifuging, in a liquid having an acid content of 25-40% and a temperature below 130 F., separating out the liquid from the cellulose nitrate and -continuously passing the nitrate through a series of acid baths of progressively decreasing concentrations approaching as the limit, 0, equilibrium conditions being at least approached in each wash liquid which is employed.

3. In the manufacture of cellulose nitrate in which cellulose is nitrated and centrifuged, the steps. which comprise immediately sousingthe cellulose nitrate, after centrifuging, in a liquid having an acid concentration of 25-40% and a temperature below 130 F., and within not more than 15 minutes applying pressure to the cellulose nitrate suiilcient to squeeze, out the major portion of the acid, and continuously passing it through a series of acid baths of progressively decreasing concentrations approaching as the limit, 0, equilibrium conditions being at least approached in each wash liquid which is employed.

4. In the manufacture of cellulose nitrate in which a cellulose is nitrated and centrifuged, the step which comprises immediately sousing the cellulose nitrate after centrifuging with aqueous sulfuric and nitric acids of 25-40% strength and at a temperature below 130 F., and within less than 15 minutes separating the acid liquid from the cellulose nitrate and washing the cellulose nitrate to substantially remove the acid therefrom.

5. In the manufacture of cellulose nitrate in which cellulose is nitrated and centrifuged, the steps which comprise immediately sousing the cellulose nitrate, after centrifuging, with an aqueous acid liquid having a' concentration 25-40% and a temperature of below 130 F., separating a portion of the acid by screeningthe mixturaseparating further'liquid by applying pressure to the-solid sufllcient to squeez out acid and then continuously passing the-cellulose nitrate through a series of aqueous acid baths of progressively decreasing concentrations ap-" proaching' as the limit, 0, equilibrium conditions being at least approached in each wash liquid which is employed. t

6. In the manufacture of cellulose nitrate in which cellulose is nitrated and centrifuged, the steps which comprise immediately sousing the cellulose nitrate after centrifuging in an aqueous acid liquid having a concentration of -40%, and a temperature below 130 F., separating a portion of the acid from the nitrate by screening the 1 mixture, separating a further quantity of acid by applying pressure to the nitrate whereby the acid is removed from the nitrate in less than 15 minutes, then washing the nitrate by continuously passing it through a series of acid baths of progressively decreasing concentrations approaching as the limit, 0, equilibrium between the acid and the nitrate being approached in each bath, pressure having been applied to the cellulose nitrate from each wash bath to squeeze out a substantialportion of the liquid held thereby before introing concentrations approaching as the limit, 0, V

equilibrium between the nitrate and acid, bath being attained in each instance, the cellulose nitrate in passing from-one bath to the next being subjected to a draining action and then to rolling pressure .to eliniinatethe major portion of the liquid in close association with the cellulose nitrate prior to its introduction into the succeeding acid baths WILLIAM E. SILLICK. 

